Apparatus and method for analysing downhole water chemistry

ABSTRACT

The invention concerns an apparatus for analysing water chemistry. According to the invention, the apparatus is adapted to operate downhole and comprises a colouring agent supply device for supplying a colouring agent to a water sample, the colour of the water sample thus supplied being indicative of the water sample chemistry, and a colorimetric analyser arranged to determine the colour of the water sample.

FIELD OF THE INVENTION

The present invention relates to an apparatus and method for downholewater chemistry analysis.

BACKGROUND OF THE INVENTION

Well operators commonly need to understand downhole water chemistry tohelp them decide production strategies and determine corrosion rates,scale formation rates, formation geochemistry etc.

More specifically, the pH and qualitative/quantitative analysis of thepresence of specific ions in downhole water are often required.

Conventionally, water chemistry measurements are performed in thelaboratory on fluid samples retrieved from below ground. However, waterchemistry is not often preservable over the temperature and pressurechanges typically induced by transportation from subterranean locationsto the surface, and so a chemistry measurement of a sample collected forlaboratory analysis will not always provide a result that can be relatedto the downhole value. Consequently, the water chemistry measured in thelaboratory may vary significantly from that existing downhole.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a more reliableanalysis of downhole water chemistry.

Accordingly, in a first aspect, the present invention provides anapparatus for analysing water chemistry, the apparatus being adapted tooperate downhole and comprising:

a colouring agent supply device for supplying a colouring agent to awater sample, the colour of the water sample thus supplied beingindicative of the water sample chemistry, and

a calorimetric analyser arranged to determine the colour of the watersample.

An advantage of the apparatus is that it allows in situ analysis to beperformed, thereby avoiding the problems associated with transportingwater samples to the surface. The present invention is at least partlybased on the realisation that colorimetric analysis is a technique thatcan be adapted for performance downhole, i.e. in relatively demandingand hostile conditions.

In one embodiment the apparatus is installed downhole (e.g. in ahydrocarbon well or an aquifer).

Preferably the calorimetric analyser is connected to a processor fordetermining the water sample chemistry from the colour of the watersample. The processor may also be adapted for use downhole, oralternatively it may be intended for remote installation e.g. at thesurface. For example the processor may be a suitably programmedcomputer.

The water sample colour may be indicative of e.g. water pH or a selectedion concentration level.

In one embodiment the calorimetric analyser comprises a spectrometer. Anadvantage of a spectrometer-based approach to colour analysis is that ithas the potential to provide fast answers to questions of pH, corrosionchemistry and scale formation, which can be crucial for deciding e.g.completion design and materials and scale treatment programs.

A further aspect of the present invention provides for the use of theapparatus of the previous aspect for in situ analysis of downhole waterchemistry.

In another aspect the present invention provides a method for analysingdownhole water chemistry, the method comprising the steps of:

(a) supplying a colouring agent to a downhole water sample, the colourof the water sample thus supplied being indicative of the water samplechemistry, and

(b) determining the colour of the water sample,

wherein steps (a) and (b) are performed in situ.

In another aspect the present invention provides a method for monitoringcontamination of downhole water, the method comprising the steps of:

(a) adding a tracer agent to a fluid which is a potential contaminant ofthe downhole water,

(b) supplying a colouring agent to a sample of the downhole water, thecolour of the water sample thus supplied being indicative of thepresence of the tracer agent, and

(c) determining the colour of the water sample,

wherein steps (b) and (c) are performed in situ.

The potential contaminant may be drilling mud filtrate. The downholewater may be either connate or injected water.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the present invention will now be described withreference to the following drawings in which:

FIG. 1 shows a schematic diagram of a Live Fluid Analyser installed on aflow line,

FIG. 2 a shows the room temperature absorbance spectra of (a) the acidform of phenol red, (b) the base form of phenol red, (c) phenol red in apH 8 solution, and (d) a weighted sum of the acid and base form spectrafitted to the pH 8 solution absorbance spectrum,

FIG. 2 b shows graphs of base fraction of phenol red (right handvertical axis) and calculated pH (left hand vertical axis) as functionsof prepared solution pH,

FIG. 3 a shows the room temperature absorbance spectra obtained from (a)phenol red in deionised water and (b) phenol red in deionised waterafter heat treatment at 150° C. for 24 hours, and

FIG. 3 b shows the absorbance spectra obtained from (a) phenol red in apH 7.4 buffer solution at 22° C. and (b) phenol red in the pH 7.4 buffersolution at 150° C.

DETAILED DESCRIPTION OF THE INVENTION

In general terms, the present invention relates to downhole colorimetricanalysis. A preferred approach for the determination of pH and detectionof the presence of specific ions involves injecting a specific indicatoror reagent into a sample of water and determining the resulting colourof the fluid with an optical spectrometer.

Ions of interest for detection include those of Ca, Ba, Sr, Al, Cl, F,Fe, Mg, K, Si, Na, and ions containing sulphur and carbon (for examplecarbonate, bicarbonate, sulphate). Use of colorimetric and spectrometricanalysis along with procedures and reagents required to determine thepresence/quantity of some of these ions have been described in theliterature (Vogel, A. I., Text-Book of Quantitative Inorganic Analysis,3^(rd) Edition, Chapter 10, John Wiley, 1961; Sandell E. B.,Colorimetric Determination of Traces of Metals, 3^(rd) Edition,Interscience Publishers, 1959). However, we propose, for the first time,the application of these methods, in a downhole environment, to theanalysis of downhole water as found in oil and gas fields, as well asaquifers. Typical temperatures and pressures found in a downholeenvironment are in the range of 125° C. and 10,000 psi, respectively;however they can go up to as high as 175° C. and 20,000 psi.

To perform quantitative measurements of pH or ion concentration, theoptical absorption of the unknown species can be determined eitherrelative to a standard solution (which could be the water sample itselfprior to indicator/reagent addition) or with a stable and previouslycalibrated spectrometer.

Desirably, the spectrometer should be capable of operating over thevisible spectrum of 400 to 760 nm, which is from ultraviolet to infraredrespectively.

In one embodiment we propose fitting a known Modular Dynamic Tester(MDT) with a Live Fluid Analyzer (LFA) module (R. J. Andrews et al.,Oilfield Review, 13(3), 24-43). The LFA would inject coloured indicatorsto the water flowing through the MDT so that pH can be determined. Itcan also add suitable reagents to the water for determination of thepresence/concentration of selected ions.

FIG. 1 shows a schematic diagram of the LFA installed on a flow line 1,the other parts of the MDT not being shown. An arrow indicates thedirection of water flow in the flow line. The LFA has an upstream dyeinjector 6 and a downstream optical analyser 2. The analyser comprises alight source 3 on one side of the flow line and a facing light detector4 on the opposite side of the flow line. When a preselected indicator orreagent 5 is injected into flow line it mixes with the water and iscarried downstream to the analyser, whereupon the detector generates asignal indicative of the colour of the water. If required a mixer, notshown in the figure, such as a double helix, can be used to promotemixing of the water and dye. A processor (not shown) then determines thewater chemistry from the signal e.g. using approaches discussed below.

Such colorimetric analysis also allows contamination of formation waterby water-based mud filtrate to be detected. This can be achieved bysuitable indicator/reagent selection such that the water-based mudfiltrate and formation water generate different respective colours.

Another option is to add a tracer ion or other species (for example,nitrate, iodide or thiocyanate ions) to the drilling fluid. A reagentcan then be used in the LFA, which produces a colour change in thepresence of the tracer so that the tracer can be detected and preferablyquantified. In this way real-time monitoring of connate water forcontamination by the filtrate can be achieved.

A possible reagent for detecting iodide is the iodobismuthite ion,formable from a solution of bismuth in dilute sulphuric acid. This iongives a yellow orange colouration and is sensitive up to 1% iodide(Vogel, A. I., Text-Book of Quantitative Inorganic Analysis, 3^(rd)Edition, Chapter 10, p803 John Wiley, 1961).

We now describe how indicator colouration can be used to measure pH.However, similar considerations apply when the colour of any reagent isbeing used to measure ion concentration.

For pH measurements the choice of indicator depends to a significantextent on the accuracy with which the pH is required. As an example, wetake a universal indicator, a volume of which has been injected into thesample flowline upstream of the optical detector. The indicator volumeis determined by the flow rate of the water and intensity of the colourand is usually a small fraction of the total volume. The universalindicator may be formed e.g. from a mixture of 0.2 g of phenolphthalein,0.4 g methylred, 0.6 g dimethylazobenzene, 0.8 g bromothymol blue, and 1g of thymol blue in 1 l ethanol. To this solution is added NaOH(aq)until the solution appears yellow. The colours of the solution as afunction of pH are listed in the table below (Vogel, A. I., Text-Book ofQuantitative Inorganic Analysis, 3^(rd) Edition, Chapter 1.30, p59 JohnWiley, 1961). pH 2 4 6 8 10 12 Colour Red Orange Yellow Green BluePurple

An alternative is to use a plurality of indicators each of which isspecific to a respective pH range. This may result in a more precisedetermination of pH.

The pH of an unknown solution may be obtained using the equation below(R. G. Bates, Determination of pH: Theory and Practice, Chapter 6, JohnWiley, 1964): $\begin{matrix}{{pH} = {{pKa} + {\log\frac{\gamma_{B}}{\gamma_{A}}} + {\log\frac{B}{A}}}} & (1)\end{matrix}$where Ka is the thermodynamic equilibrium constant for the indicator andis a function of temperature; A and B are the respective fractions ofthe acid and base forms of the indicator; and γ_(A) and γ_(B) arerespective activity coefficients of the acid and base forms of theindicator, and depend on ionic strength of the solution and temperature.Both Ka and activity coefficients could be weak functions of pressure aswell.

The fraction of the indicator that exists in the acid form (A) and baseform (B) may be measured spectroscopically. The absolute concentrationof the dye does not appear in the equation and hence the pH calculationis independent of the volume of dye injected or the flow rate of thewater stream as long as the concentration is such that Beer's law issatisfied. The functional dependence of Ka on temperature (T) has beenstudied and measured for a number of reactions and a general equationthat can describe this dependence is (D. Langmuir, Aqueous EnvironmentalGeochemistry, Chapter 1, Section 1.6.2, Prentice Hall, 1997):$\begin{matrix}{{\log\quad{Ka}} = {a + {bT} + \frac{c}{T} + {d\quad\log\quad T} + {{\mathbb{e}}\quad T^{2}}}} & (2)\end{matrix}$

The parameters in this equation may be obtained by calibration in thelaboratory over the desired temperature range using standard buffers ofknown pH. Dependence on pressure may also be obtained throughexperimental calibration if necessary. Several models have been proposedfor activity coefficient estimation. For example, the Debye-Huckelequation is commonly used for low ionic strength solutions and thePitzer model at higher ionic strengths (D. Langmuir, AqueousEnvironmental Geochemistry, Chapter 4, Section 4.2, Prentice Hall,1997). Ionic strengths can be derived from downhole water sampleconductivity/resistivity measurements as is done in the MDT oralternatively from other wireline measurements such as resistivity logs.For very dilute solutions and/or for acid and base forms that havesimilar behaviours, the activity coefficient term may be neglected. Thusequation (1) provides a means for determining pH under downholeconditions for most temperatures, pressures and ionic strengthsencountered in practice.

As an example, FIG. 2 a shows the room temperature absorbance spectra of(a) the acid form of phenol red and (b) the base form of phenol red. Theacid form has a peak at about 432 nm and the base form at about 559 nm.FIG. 2 a also shows (c) the measured absorbance spectrum of phenol redin a pH 8 solution, and (d) a weighted sum of the acid and base formspectra fitted to the measured absorbance spectrum, the weightingsproviding the base and acid fraction of phenol red in the pH 8 solution.

Similar analyses can be performed for solutions prepared with differentpH levels. FIG. 2 b shows a graph of base fraction of phenol red (righthand vertical axis) as a function of prepared solution pH (horizontalaxis). Using equation (1) it is then possible to, calculate the pH ofeach solution. The calculated pH values (left hand vertical axis) arealso plotted on FIG. 2 b. They show that, in this example, pH determinedby spectroscopy is highly accurate for phenol red base fractions in therange of about 0.05 to 0.95 corresponding to pH values from 6.5 to 9.The range of pH measurement can be increased to 6 to 9.5 if the acid andbase fractions can be spectroscopically detected at lower levels of0.02.

The accuracy of the pH measurement is higher when the pH is close to thepKa value and decreases when the pH departs from the pKa. Thus, if thelikely pH range is known, an indicator can be selected which has a pKavalue such that a desired level of accuracy can be achieved. Acombination of indicators may be chosen to cover the pH range typicallyexpected in formation waters. In this way, provided the optical analyserhas suitable wavelength windows to observe the colour changes, the pHcan be obtained to within a value of a few tenths. Depending on how theindicators interact with each other, multiple injectors in series orparallel may be used for the different indicators or a single injectorwith a mixed indicator solution may be deployed.

The analysis may be performed using a stable and calibratedcolorimeter/spectrophotometer. Alternatively, the absorbance spectra ofthe water sample in the flow line prior to indicator injection can yieldthe baseline. Yet another option is to use a reference solution tocalibrate the colorimeter/spectrophotometer. The last two optionsprovide a means of compensating for any possible inherent water colour.

Further improvements may be obtained if a series of buffer referencesolutions are supplied, each differing in pH e.g. by about 0.2 andcovering the range around the expected pH value. Indicator is then addedto known volumes of the buffer solution and the water sample and thecolours compared to determine the pH. To ensure accuracy, preferably thewater sample is a captured sample.

For downhole use, the indicator should be stable and chemically activeat the temperatures expected downhole. As an example, FIG. 3 a shows theroom temperature absorbance spectra obtained from (a) phenol red indeionised water and (b) phenol red in deionised water after heattreatment at 150° C. for 24 hours. The heat treatment results in only a10% loss in absorbance, demonstrating that the phenol red indicator cansurvive prolonged exposure to temperatures of up to 150° C.

However, it may be necessary to calibrate each indicator/reagent for thedifferent temperatures and ionic strengths to which it will be exposeddownhole. FIG. 3 b shows the spectra obtained from (a) phenol red in a7.4 pH buffer solution at 22° C. and (b) phenol red in the 7.4 pH buffersolution at 150° C. At 150° C. the phenol red is still chemicallyactive, the increase in base fraction at the higher temperature beingdue to changes in pKa and the pH of the buffer solution withtemperature.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

1. An apparatus for analysing water chemistry, the apparatus beingadapted to operate downhole and comprising: a colouring agent supplydevice for supplying a colouring agent to a water sample, the colour ofthe water sample thus supplied being indicative of the water samplechemistry, and a colorimetric analyser arranged to determine the colourof the water sample.
 2. An apparatus according to claim 1 which isinstalled downhole.
 3. An apparatus according to claim 1 wherein thecolorimetric analyser is operably connected to a processor whichdetermines the water sample chemistry from the colour of the watersample.
 4. An apparatus according to claim 1, wherein the calorimetricanalyser comprises a spectrometer.
 5. An apparatus of claim 1, whereinsaid apparatus is used for in situ analysis of downhole water chemistry.6. A method for analysing downhole water chemistry, the methodcomprising the steps of: (a) supplying a colouring agent to a downholewater sample, the colour of the water sample thus supplied beingindicative of the water sample chemistry, and (b) determining the colourof the water sample, wherein steps (a) and (b) are performed in situ. 7.A method for monitoring contamination of downhole water, the methodcomprising the steps of: (a) adding a tracer agent to a fluid which is apotential contaminant of the downhole water, (b) supplying a colouringagent to a sample of the downhole water, the colour of the water samplethus supplied being indicative of the presence of the tracer agent, and(c) determining the colour of the water sample, wherein steps (b) and(c) are performed in situ.
 8. (canceled)